<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
          *{
            padding: 0;
            margin: 0;
        }
        .box{
            width: 900px;
            height: 600px;
            border: 1px solid red;
            margin: 0 auto;
            justify-content:center;
        }
        .first{
            width:200px;
            height:200px;
        }
    </style>
</head>
<body>
        <div class="box">
                <div class="first">1</div>
                <div class="second">2</div>
                <div class="third">3</div>
            </div>
</body>
</html>